1. Field of the Invention
The invention relates to a method for avoiding the spontaneous ignition of combustible dusts in process off-gases, and to a device for carrying out the method.
2. The Prior Art
Off-gases can be from installations in which thermal processes are carried out, such as for example firing installations, combustion furnaces or melting furnaces, These off-gases may contain dusts which, before they are discharged to the environment, generally have to be retained by means of filtration and if appropriate passivation. In particular metal oxide and/or semimetal oxide mixtures in medium oxidation states are highly reactive industrial dusts. These dusts may have spontaneous ignition temperatures of less than 100° C. and tend to cause filter fires or dust explosions in downstream off-gas filters, for example. In de-dusting facilities in which, by way of example, the off-gas is blown tangentially into a cyclone-like housing as in a cyclone separator, predominantly relatively large particles are deposited on the housing inner wall. The finer particles are separated out by means of a downstream filter, for example, a fabric or fiber filter. Filter apparatus of this type often cannot be used, on account of high process temperatures, sparks flying and spontaneous ignition of highly reactive dusts.
In plants for producing high-purity silicon ingots in single crystal form using the Czochralski crucible pulling process, silicon fragments, if appropriate with the addition of dopants, such as for example antimony, arsenic, boron or phosphorus, are placed in a heatable quartz crucible and melted. A crystal ingot in single crystal form grows at the end of a seed crystal as a result of the seed crystal being dipped into the melt and rotated and pulled in the vertical direction.
As a result of the contact with the surface of the quartz crucible, oxygen is supplied to the molten material. This oxygen is distributed within the molten material as a result of the convection caused by the supply of heat and the rotation. As a result, oxides of the constituents of the molten material are formed, these oxides evaporating via the surface of the molten material. In addition to various silicon oxides SiOx (x=0 to 2), in this way in particular the oxygen compounds of antimony and arsenic, as well as phosphorus, on account of their high vapor pressure, and to a certain extent also of boron, are released.
Since crucible pulling installations generally have shielding gas, preferably argon, flowing through them under a slight vacuum, the oxides of the constituents of the melt are discharged in different oxidation states into the off-gas stream from the crucible pulling installation. These finely distributed metal oxide/semimetal oxide mixtures in different oxidation states are highly reactive, ignite spontaneously and are toxic. These industrial dusts tend to cause filter fires or dust explosions in particular in the downstream off-gas filters.
DE 3603511 C2 describes a method and device for removing pollutants in dust and gas form from off-gases, in particular off-gases produced in the manufacture of optical waveguide preforms. In this manufacture, in a first stage, dust separation takes place by means of a dry fabric filter, which is preferably coated with polytetrafluoroethylene, In one or more further stages the gaseous pollutants from the dust-free off-gas are absorbed by means of an aqueous sodium hydroxide solution as absorption liquid which is flowing in an absorption liquid circuit.
In the case of the filtration of SiOx dusts by means of fabric filter elements, these elements, on account of their construction, cause glowing pockets and fires in the folds. This leads to the filter material being damaged and to the SiOx dust passing into the downstream pumps or into the environment via the pump off-gases. To keep the process parameters pressure and throughflow constant over the entire running time of the process, a very large filter surface area is required when using these filters. High gas flow rates at a low pressure are not possible, and consequently the choice of suitable process parameters for pulling the crystals is limited.
DE 19854235 A1 describes a method for passivating combustible, metallurgical dusts which are produced in the off-gas filter of crucible pulling installations, in particular of crucible pulling installations for pulling silicon single crystals. In these installations, the dusts in the off-gas stream from the crucible pulling installation are continuously passivated by means of a reaction gas at temperatures from 50 to 500° C. For continuous passivation, a reaction chamber with a heating device and at least one reaction gas inlet is introduced between the off-gas line of the crucible pulling installation and the off-gas filter. The temperature of the reaction chamber and the supply of reaction gas into the reaction chamber are regulated as a function of the crystal pulling conditions. The reaction gas used is air, oxygen, ozone or mixtures of these gases.
The passivation by means of a continuous feed of a reaction gas has the drawback that the reaction gas partial pressure in the gas chamber of the crucible pulling installation rises. The diffusion of oxygen back into the crucible pulling installation has an adverse effect on, for example, the conductivity of the silicon single crystal ingot. In order not to significantly impair the vacuum and shielding gas flow conditions required for the crucible pulling process and the oxygen content in the crucible pulling installation, the capacity of the oxidation device is limited in process engineering terms. Incomplete oxidation can cause highly reactive, spontaneously igniting and toxic oxides of the constituents of the melt to penetrate into the dust filter. This will produce glowing pockets and fires in the filter material and dust explosions, for example when the installation is being cleaned.
DE 3705793 describes a filter apparatus for gas cleaning and detoxification of off-gases from thermal processes, for example firing operations, combustion furnaces, melting furnaces or drying installations. In these thermal processes, dust and gaseous pollutants are removed from the off-gases by means of a combination of a dust filter which is insensitive to heat and flying sparks and a catalyst element in a single structural unit. These catalyst elements preferably consist of a large-cell foamed ceramic which is coated with catalytically suitable metal compounds or pure metals.
This catalyst, which is used to convert gaseous pollutants, has to be applied and fixed to the support material made from ceramics, woven fabrics or nonwovens made from high-temperature-resistant material, such as quartz fibers or stainless steel fibers. The dust filter elements are mounted upstream of the catalytically coated foam ceramic elements for subsequently converting the gaseous pollutants. The filter elements are difficult to clean, which in the case of highly reactive, spontaneously ignitible and toxic dusts, such as for example oxide mixtures of silicon doped with arsenic, phosphorus or antimony, leads to a high level of outlay in the filter cleaning infrastructure. This is in order to minimize risks to the health and safety of the cleaning staff.